Auricular electroacupuncture in rats is found to increase levels of beta-endorphin-like radioreceptor activity in rat cerebrospinal fluid concomitant with increases in pain thresholds. Acupuncture also induced a significant decrease in endorphins in the basomedial hypothalamus, periaqueductal gray matter and medial thalamus. These findings indicate that acupuncture is capable of activating the endorphin systems in several brain regions concerned with pain processing. Foot-shock stress also had a similar effect while cervical probing in female rats decreased endorphins only in the periaqueductal gray matter and the septum. Type I opiate receptors were found to increase along a hierarchical gradient in both the visual and auditory processing systems in the monkey cortex. Type II opiate receptors were distributed evenly among all cortical regions. Type I opiate receptors appear to mediate the depressant actions of opiates in brain while Type II opiate receptors mediate the excitatory and euphorogenic actions.